Best Super Powers for Custom Race?

I'd say all compares to your style of playing.

I like the first strike super ability, gives you a true advantage if played right.
I'm now playing with the Yor super ability, wich makes my defenses very good. They can only move 3 steps in my influence.
The spore ability is ok, fun to toy around with.
The drath ability to get everybody into a war is fun as well.
Super diplomat works as well.

SuperStrats i dont really like all to much are drengin (the ships you get are useless), Super adapter (it would be OK if they could colonize all special worlds), Super spy.

The others are OK

Again, its all about how you want to play.
If you want a laid back approach, just playing and not to much war i'd suggest terran or drath super ability.
Want to war, i'd say go for the spore or first strike approach.

Other than that, 8 more options to go

Personally i like Breeder and Hive. Both make the early part of the game a little easier and both set you up well economically. For a quick easy kill on a small map, go for Spore.

If you are playing for anything other than a conquest victory, then i would go with Diplomat.

First strike is the best ability IMO. If playing aggressively obviously it's a big advantage allowing you to crush your opponents. Meanwhile if playing defensively (e.g. going for research and/or influence victory) it's also invaluable since if you do end up at war with someone you can hold them off with a tiny force where otherwise you would have been crushed. Obviously if you plan to never be at war with anyone then it's worth going for another ability, but I find such situations fairly rare (although it could be due to my play style)

I have to agree that amongst the worst is the Drengin super dominator. Early on, you only get a very few ships. Later on, the ships are all but useless.

As for best, as mentioned above, it depends on your play style, but I think that Diplomat and First Strike are both VERY powerful.

Diplomat and Warrior and Breeder and Annihilator are all top notch abilities. Manipulator and Organizer and Isolationist and Adapter are second tier. Spy and Dominator and Trader just seem fairly lackluster.

All IMHO, of course. And as for why you should take my opinion seriously - well, you probably shouldn't. I'm just some guy on the internet. But still, that's how I rank them.

Or are you?

You expect me to know?